Mesh Upload - Did not find a material to be used with texture

If I remember correctly, this is not an error, but a warning. This normally happens under one of the following conditions:
1) You are uploading multiple objects at once, and they use different textures. So if you are uploading objects A and B, with textures 1 and 2 respectibly, A is going to generate a warning saying that texture 2 is not recognized; and B is going to say the same about texture 1
2) The material and texture names are not aligned to the documentation requirements. Here's the link to the docs: https://developers.meta.com/horizon-worlds/learn/documentation/custom-model-import/creating-custom-models-for-horizon-worlds/materials-guidance-and-reference-for-custom-models

Being a warning this will not stop you from uploading, and the scenario 1 is not really a problem. But if your warning is generated by the second scenario, then yes, this is a problem as the mesh will not render with the texture in Worlds. The docs contain guidance on this.